After-tax 401(k) Plan

An after-tax 401(k) contribution is made with post-tax dollars, but unlike a Roth 401(k), the earnings on the account are taxed as ordinary income upon withdrawal. Only about 23% of employers offer this feature. By itself, the after-tax sub-account is an inferior vehicle compared to a Roth 401(k). Its one strategic role, however, is irreplaceable: it is the launchpad for the Mega Backdoor Roth strategy.

• The contribution limit for all defined contribution plans combined is $72,000 in 2026.
• You can contribute up to $35,250 to your after-tax 401(k) after accounting for your $24,500 elective deferrals and an assumed 50% employer match ($12,250) on a high salary.
• By executing an immediate in-plan Roth conversion or rolling the after-tax balance out to a personal Roth IRA, you sweep these assets into a tax-free wrapper before any taxable earnings accrue.
• Highly recommended: if your employer’s plan supports this feature, max it out immediately.

Mega-backdoor Roth IRA

If your income is too high to contribute directly to a personal Roth IRA, the Mega Backdoor Roth is your primary bypass. This strategy leverages the after-tax 401(k) sub-account to fund massive tax-free assets.

The conversion mechanics are straightforward:

1.

Make non-deductible after-tax contributions to your 401(k) up to the plan’s maximum.

2.

Convert those contributions immediately to your Roth 401(k) within the plan (an in-plan Roth conversion) or roll them out to a personal Roth IRA via an in-service distribution.

3.

Because the contributions are converted immediately, they have no time to accumulate earnings. Thus, the conversion triggers $0 in tax liability.

4.

The IRA aggregation rule of IRC §408(d)(2) does not reach this transaction — your traditional, SEP, and SIMPLE IRA balances are irrelevant, which is the structural advantage over the ordinary backdoor Roth.

Be precise about which pro-rata rule is switched off, because a different one still applies inside the plan. Under IRC §72(e)(8), a distribution from the after-tax sub-account normally carries a proportionate slice of its own accumulated earnings, which are taxable on conversion. Two things defeat it: converting immediately, so there are no earnings to pro-rate, and a plan that maintains genuine separate accounting for the after-tax source. Notice 2014-54 is what permits the clean split — basis to the Roth, earnings to a traditional IRA — when earnings have accrued. The practical rule: set the in-plan conversion to trigger automatically on every after-tax payroll deposit. A plan that only sweeps quarterly leaves earnings to accumulate and turns a $0 conversion into a taxable one. You will receive Form 1099-R indicating the distribution/rollover and report it on your individual return; no Form 8606 is required here, since that form tracks IRA basis and belongs to the regular backdoor Roth (section “Backdoor Roth IRA”), not an after-tax 401(k) conversion. Once the funds reside in your Roth IRA or Roth 401(k), they are completely free of Required Minimum Distributions during your lifetime (matching the treatment under current law). For reporting details, see section “Tax Reporting of Mega-backdoor Roth IRA”.